This invention is generally in the field of electron emission based techniques, such as electron microscopy (EM), specifically scanning electron microscopy (SEM), and electron beam lithography (EBL), utilizing a nanotube-based electron emission device, with high electron optical quality, specifically high brightness and low energy spread. The electron emission device of the present invention can be used with any electron beam column or other system that requires such properties.
It is the common goal of various applications utilizing an electron beam source, such as SEM and EBL, to have a simple and stable electron beam source, in particular, a source capable of being installed in a miniature device and having high electron optical quality. The simplicity of the electron source is defined by its working conditions, such as operating vacuum and temperature parameters. As for the optical quality, it is predominantly determined by the brightness (i.e., current density per solid angle) and the energy spread of the electron beam. Both the brightness and energy spread determine the amount of current that can be focused on a small spot on the surface of a sample,
SEM and EBL are known techniques widely used in various applications, such as the manufacture of semiconductor devices. Electron sources conventionally used in SEM and EBL tools are typically one of three kinds: thermal sources, cold field emitters (CFE), and thermal field emitters or xe2x80x9cSchottky-emittersxe2x80x9d.
An electron beam generated by a thermal source has a wide energy spread, i.e., about 2 eV for tungsten filaments and 1.2 eV for LaB6, and low brightness, i.e., about 105-106 A/cm2sr. Consequently, the electron sources of this kind require the complicated construction of an electron beam column, as well as high acceleration voltage, to achieve the resolution of a 1-10 nm.
CFE are characterized by a higher brightness (about 108-109 A/cm2sr) and a narrower energy spread (about 0.3-0.4 eV), as compared to that of thermal sources. It is usually made of single crystal tungsten. Such a cathode-electrode requires xe2x80x9cflashingxe2x80x9d at a high temperature (more than 1800 K) to clean and reform its surface. Moreover, CFE suffer from ultra-high vacuum requirements (e.g., 10xe2x88x929-10xe2x88x9210 Torr), which is a major factor in the cost of a CFE-based device. Due to the unavoidable adsorption of molecules on the tip-like cathode-electrode, it is complicated to operate CFE in a stable manner.
A TFE source utilizes a compromise concept between those underlying the implementation of the electron sources of the above two kinds. TFE are made of tungsten coated by zirconium oxide, aimed at lowering the work function. These sources, as compared to CFE, are more stable, require lower vacuum (about 10xe2x88x928-10xe2x88x929 Torr), and have a comparable high brightness (about 107-108 A/cm2sr). A TFE ""source still requires an ultra high vacuum. A further drawback is associated with the need to stabilize the system at a high working temperature, i.e., about 1800xc2x0 C. The voltage required to achieve a desirable high resolution is typically high. This may damage the sample and/or cause undesirable charging thereof To solve this problem, beam deceleration may be employed. However, this complicates the construction of an electron beam column, and enhances chromatic aberrations. All the conventional electron sources use a very small fraction of the current that is emitted from the electron source. A state of the art Schottty source emits about 100 xcexcA/sr with only 10 nA of the source current in a TFE gun can actually be used as a probe current.
As a result of the above disadvantages of the conventional electron sources, their use in EM and EBL tools make these tools expensive and bulky. This impedes their application as integrated tools. Indeed, when using SEM for the inspection of workpieces on a production line, for example, in the manufacture of semiconductor devices, SEM is typically a stand-alone machine accommodated outside the production line. Accordingly, workpieces to be inspected are removed from the production line and brought to the SEM. This slows the production. Moreover, any unnecessary handling of such delicate workpieces as semiconductor wafers is undesirable. Thus, it is highly desirable to use a miniaturized SEM that can be brought to the sample to be inspected, rather than bringing the sample to the SEM. The miniaturized SEM technology is known as xe2x80x9cMicro-columnsxe2x80x9d. As for the lithography tools, it is a core challenge of the semiconductor industry, to go beyond optical resolution, which currently limits the minimal feature size of the active elements of a semiconductor device.
Electron beam lithography is not limited by the optical diffraction limit, but by the throughput of the electron beam apparatus. There are three main approaches to this problem. First, the use of a miniature electron beam source device that can be utilized in an arrayed operation (xe2x80x9cmicro-columnsxe2x80x9d); second, direct writing in xe2x80x9cproximity focusxe2x80x9d; and third, the SCALPEL (Scattering with Angular Limitation Projection Electron Beam Lithography). Although these technologies have been known for several years, none of them is used in commercial applications. This is due to the following reasons: the miniaturized arrayed operation is limited by the absence of an adequate electron source, which has the desired electron optical quality (high brightness and low energy spread), which is compatible with miniaturization and with silicon technologies, and which can be produced with sufficient alignment to the optical axis. Currently micro-columns utilize a TFE. The high temperature of the TFE sources places additional limitation on the micro-columns. The proximity focus electron-beam writing was not utilized due to the absence of an electron source that can be patterned in the sub-100 nm scale and that emits electrons with the required electron-optical quality, specifically, a sufficiently narrow angular distribution. The SCALPEL technology is limited by several factors, notably, the small area that can be uniformly exposed by the currently available electron sources.
Attempts have been made to develop electron beam sources with improved electron-optical quality and operating vacuum parameters, so far with no success. Concurrently, carbon-based nanotubes have been developed and studied as field emitters. Their main properties and advantageous features are disclosed, for example, in the following publications:
Shoushan Fan et al., xe2x80x9cSelf-Oriented Regular Arrays of Carbon Nanotubes and Their Field Emission Propertiesxe2x80x9d, Science, Vol. 283, p. 512-514, January 1999;
O. H. Wang et al., xe2x80x9cField Emission from nanotube Bundle Emitters al Low Fieldsxe2x80x9d, Appl. Phys., Lett., 70 (24), pp. 3308-3309, June 1997;
J. M. Bonard et al. xe2x80x9cField Emission Induced Luminescence from Carbon Nanotubesxe2x80x9d Phys. Rev. Lett., 81, 1441, 1998;
Phillip G. Collins and A. Zetti xe2x80x9cA Simple and Robust Electron Beam Source from Carbon Nanotubesxe2x80x9d, Appl. Phys. Lett., 69 (13), pp. 1969-1971, September 1996;
Walt A. de Heer et al., xe2x80x9cA Carbon Nanotube Field Emission Electron Sourcexe2x80x9d, Science, Vol. 270, November 1995;
O. G. Wang et al. xe2x80x9cA Nanotube-Based Field-Emission Flat Panel Displayxe2x80x9d, Appl. Phys. Lea., Vol. 72, No. 22pp. 2912-2913, 1998;
WO 98/11588; WO 96/42101; EP 0913508; U.S. 5,973,444; WO 98/05920; and U.S. Pat. No. 5,872,422;
Various molecular morphologies can be grown, known as MWNT (Multi Wall Nano Tubes) and SWNT (Single Wall Nano Tubes). MWNT may be produced as capped or open, and SWNT can appear also as tight bundles. The various nanotubes have been grown with diameters down to a few nanometers.
Although nanotubes are known to have exceptionally good field emission properties (high current at low applied voltage, as well as low energy spread) they did not find their application in EM or EBL. This is largely because the brightness of xe2x80x9cbarexe2x80x9d carbon nanotubes is essentially low as compared to that of a TFE source. Thus, it is essential to design a gun with superior optical properties that will utilize the elevated field-emission properties of the nanotubes.
According to a technique disclosed in the article of Saito et. al., Nature, vol. 389, 9th October 1997, pp 554-5, carbon nanotubes were suspended against an anode plate to obtain a light spot on a fluorescent screen. The source""s virtual angular divergence observed was of the same order of magnitude as that of a conventional TFE source, but the total emitted current is lower. This is a strong indication that simply replacing a conventional electron source with a nanotubes-based source does not constitute a satisfactory improvement of the brightness of the source.
As indicated above, the requirement for a miniature electron source is more essential with the development of the micro-column. Prior art techniques of the kind specified disclosed, for example in the articles of Kratchmer et al, JVS, B13, 2498 (1995) and Yu et al, JVS B13, 2436 (1995), and in U.S. Pat. No. 6,023,060, use a xe2x80x9cpointxe2x80x9d electron source which, in principle, emits a diverging laminar electron beam, which is then xe2x80x9ccroppedxe2x80x9d at an aperture (or through a few apertures). These techniques utilized either a CFE or a miniaturized Schottky TFE.
Several publications disclose the emission properties of a single nanotube. The current, I, vs. voltage, V, properties of a single nanotube has been disclosed by Saito et al, App, Surf. Sci. 146,305(1999), Dean et al, J. Appl. Phys. 85, 3832(1999) and Rinzler et al Science, 269,1550(1995). In all these experiments, a few nanometers thick nanotube (of various morphology) was attached to a thin conductor and suspended vertically against an anode plate. In these experiments the distance between the anode and the cathode-tip ranged from about one millimeter to a few centimeters.
In order to compare between the experiments one needs to compare the current vs. the electric field on the tip, the latter being determined by the applied voltage and the experimental setup. If one assumes that the reference electric field is the fixed electric field between two parallel plates separated by a distance, d, which is equal to the distance between the end of a tip and a flat anode plate, then one finds by simple calculation that the electric field on the tip of a very long (to be considered infinite or suspended) and a few nanometers diameter nanotube is enhanced relative to V/d by about two orders of magnitude. In all the above experiments, V/d vs. I may be compared, while taking the field enhancement and the material work function as having the same order of magnitude. One finds that in all the above independent experiments, the current appears at a threshold value of V/d and that the best performing types of nanotubes produce approximately 500 nA at V/d? 0.05 V/xcexcm. In all cases, a saturation current is obtained. From the results of Saito et al, Nature 389, 554(1997), the angular divergence of the electron beam can also be estimated, as a spot of about 1 cm diameter is seen on a fluorescent screen at a distance of 6 cm. This gives an angular current density of about 40 xcexcA/sr for a maximum current of 900 nA.
Thus, a single nanotube device can produce a relatively large current for a very low extracting voltage at a distance between a nanotube-tip and anode plate of the order of 1 xcexcm, but the angular current density will be too small to simply replace the TFE gun utilized within the known designs of micro-columns,
U.S. Pat. Nos. 5,773,921 and 5,973,444 and EP0913508 disclose various techniques of micro-fabricating a single nanotube and a plurality of nanotubes on a conducting substrate. In all of these field emission (FE) devices consisting of one or more nanotubes attached to a conducting substrate and separated by an insulating layer, a low voltage extracting gate is utilized. These designs are to be considered xe2x80x9cbarexe2x80x9d, as they simply provide a beam governed by the initial angular divergence. From the angular current density calculated above, it is clear that this device is insufficiently bright as an electron gun for a SEM, EBL or micro-column. As a specific example, if one applies the scaling rules developed by Chang et. al., JVS B7, 1855(1989), to a single nanotube device of this type, it is clear that this device is not appropriate as an electron source in a dual immersion lens electron gun device.
There is accordingly a need in the art to improve electron source based devices, designed to provide a high brightness and low energy spread, utilizing one or more nanotubes as field emitters.
It is a major feature of the present invention to provide such an electron emission device, which is sufficiently miniature and provides a beam with very high electron optical quality, as required for electron microscopy and other applications. In particular, to improve the brightness of the source the initial angular divergence of the electron beam is to be decreased, as compared to that obtained with the devices of the kind specified disclosed in the above prior art publications.
Generally, the design concept proposed in this invention for reducing the angular divergence of the beam can be applied to other miniature electron sources or macroscopic point sources, and is not limited to nanotubes.
It is a further feature of the present invention to provide such a device, which enables various, important, novel applications of the electron beam source.
A central idea of the present invention is based on the implementation of an electron source with a cathode-electrode which is formed by one or more nanotubes within a specially designed environment, and is associated with one or more gates formed by an anode electrode. According to the technique of the present invention, this is done in a way that minimizes the angular divergence of the beam to the extent that the invention discloses a gun that produces a nearly laminar electron beam. This electron source allows a significant fraction of the current that is emitted from the gun, to be used in a probe. Such an electron source may be used in an electron microscope, either for scanning or transmission purposes, in a lithography tool, in a field emission display and for direct writing purposes.
The nanotube-based electron gun is expected to require lower vacuum conditions (about 10xe2x88x926-10xe2x88x927 Torr), as compared to that required by conventional electron sources. The relaxed vacuum requirement is due to the fact that nanotubes are less sensitive to adsorption than conventional emitters. Another reason for the lower vacuum requirement is the xe2x80x9cpoint on a planexe2x80x9d configuration: the nanotubes are situated on a conducting surface, the lines of the electric field are effectively directed between the two planes. Thus, the tendency of ions to be accelerated towards the emitter is significantly decreased.
According to the gun design proposed in the invention, the nanotubes can be assembled in a way that provides very high brightness. The high brightness, combined with the low energy spread (about 0.1 eV), enables a resolution of less than 1 nm to be provided with relatively low accelerating voltages. Since the brightness of the beam, as it exits the electron gun, is high, an optical column attached to the gun can be significantly shorter than the micro-columns known so far. Since heating of the tip-electrode to high temperatures is not required and since the emission drift is small, a carbon nanotube-based bright electron source has obvious advantages.
It is important to note that using one of the nanotubes growing techniques disclosed in the literature (e.g. CVD growth, as disclosed in Shoushan Fan et al., Science, Vol. 283, p. 512-514, 1999 for dense nanotubes, or the technique disclosed in Z. F. Ren., App. Phys. Lett., Vol. 75, p. 1086-1088, 1999 for a single nanotube), the manufacture of such an electron source device can be built in a compatible way to the known silicon processes. The nanotubes are grown self-aligned to an extraction electrode. In a micro-column, they can also be aligned with the other column components. The desired dimensions and optical properties of the gun can be readily determined by charged particle dynamics simulation, which take into account space-charge effects (TRICOMP, by Field Precision, Albuquerque, N. Mex., USA).
Due to the above advantages of the electron source device, its use in a SEM in the manufacture of semiconductor devices will significantly increase the SEM""s throughput, due to the shorter pumping time of a sample and due to the fact that the inspection can be done in-situ. It will further increase the throughput due to the ability to provide an arrayed (parallel) operation of a number of columns.
Thus, according to one aspect of the present invention, there is provided a device to be applied to a sample for processing it by an electron beam, the device comprising an electron source device that comprises an electrode in the form of a first conductive layer carrying at least one electrons emitting fiber located inside a crater formed in said first conductive layer for collimating the electron beam produced by the fiber, and an extracting electrode insulated from the first layer, the extracting electrode being formed with at least one aperture located above the crater, the device thereby providing a desired angular divergence of the electron beam.
The term xe2x80x9cprocessingxe2x80x9d used herein signifies any treatment that can be applied to a sample by means of an electron beam, such as monitoring, inspection/review, patterning (including mask-making), excitation (e.g., causing the sample""s luminescence), etc. The device can be used for an electron microscope (e.g., SEM), a lithography tool, a field emission display, etc. The term xe2x80x9ccraterxe2x80x9d signifies a groove made in the first electrically conductive layer, the electrons emitting fiber projecting from the bottom of the groove. The term xe2x80x9cdesired angular divergence of the beamxe2x80x9d signifies a significantly reduced angular divergence, as compared to that of the prior art devices of the kind specified. The electron-emitting fiber is preferably a nanotube.
The electron source device may be attached to an electron beam column, which can be a miniaturized electron optical system, such as the micro-column. The provision of the electrons emitting fiber inside the electrically conductive crater enables to collimate the emitted beam, thereby significantly increasing the brightness at the exit of the electron source device. Because of the high brightness at the exit of the specified electron gun, the micro-column may be simplified and further miniaturized. For example, it is expected that no further collimation will be required after the gun. Therefore, the device may consist of the gun, the scanner, the detector, and possibly fewer lenses.
The first conductive layer may carry a plurality of electrons emitting fibers (e.g., nanotubes), wherein either several fibers are located in the same conductive crater, or an array of such conductive craters is provided, each for carrying a corresponding one of the fibers.
The ability to control the growth pattern of the nanotubes provides a novel application of the electron source device in a lithography tool for producing the desired features of a semiconductor device on the sample (e.g., wafer), in proximity focus. In other words, the nanotubes are grown on catalysts that are shaped in the form of the desired pattern. The conductive regions, where the catalysts are placed, are built as a two-dimensional cross-section of the conducting crater design disclosed in the current invention, namely, with a specific construction to reduce the angular divergence of the beam.
There is thus provided, according to another aspect of the present invention, an electron microscope to be applied to a sample, the microscope comprising an electron source device and an electron beam column, wherein the electron source device comprises an electrode in the form of a first conductive layer carrying at least one electrons emitting fiber located inside a crater formed in said first conductive layer for collimating the electron beam produced by the fiber, and an extracting electrode insulated from the first layer, the extracting electrode being formed with at least one aperture located above the crater, the device thereby providing a desired angular divergence of the electron beam.
According to yet another aspect of the present invention, there is provided a lithography tool for processing a sample to form its surface with a predetermined pattern, the tool comprising an electron source device having a plurality of electrons emitting fibers producing a plurality of beams each corresponding to one of the pattern features, the beams being spatially separated and aligned in a manner corresponding to the alignment of the pattern features to be obtained on the surface of the sample. The sample itself may serve as an extracting electrode.
More specifically, the present invention utilizes nanotubes as electrons emitting fibers, and is therefore described below with respect to this application.